The present invention relates to pressure sensitive adhesives, and more particularly to adhesives used in the graphics industry.
Pressure sensitive adhesives for the graphics industry are usually coated on one side or two sides of a substrate. The substrate can be a thin film (e.g., "tape") coated on either one or both sides, or a board (e.g., paper, plastic, or foam) coated on one side to be used for mounting. One might adhere an art poster to a board prior to framing or use a double adhesive sided tape to make a display of pictures or articles where the two sided tape is hidden between the two objects being held together. Alternatively, the tape can already be adhered on one of the substrate, with a release liner, so the release liner can be peeled away to expose a tacky pressure sensitive adhesive. A two-sided tape can be provided in a small one-half inch by three yard roll, on a dispenser, for the user to roll off small strips, place them on the back of an item, then adhere the item to another substrate. The item could be pressure sensitive wallpaper or vinyl letters for a boat name or identification number.
Both skilled as well as unskilled users of conventional pressure sensitive adhesives have been victim to sticking down a misaligned item, due to prematurely letting go of it. Even after correct placement, the user must "smooth out" the item. The further skill of achieving an acceptable bubble-free and wrinkle-free lamination is very difficult to perfect, especially for the novice. For this very reason, a product that is non-tacky until heated was developed for a number of applications. This heat activated adhesive is used extensively in the framing industry (dry mount adhesive). A significant source and amount of heat (a heat press) is used to press and heat the substrate until the adhesive melts 160 F-250 F and bonds the substrate to the item.
Heat activated adhesives have a number of drawbacks. All substrates will curl to some extent when heat and pressure are applied. The danger of burns is ever-present. The cost of electricity and fairly long production time to heat up and cool down bonded substrate are clear disadvantages. Once an adhesive becomes a liquid or soft enough to flow and effect a bond, there is a risk it will change the visual appearance of a paper or poster due to melt-through or paper/plastic transmission variables. Many applications simply are not practical for heat activated adhesives. Universal adhesion to, e.g., plastic, paper, metal, and fabric with a single adhesive is very difficult to achieve. Due to this limitation the industry has responded with many heat activated products based on polyester, polyolefins, nylons, and polyurethane, etc. Each such adhesive has its unique application temperature, melt parameters, and substrate suitability.
Certainly conventional pressure sensitive adhesive have a convenience advantage over heat activated adhesives, but the previously stated problems with such pressure sensitive adhesives, limits their appeal to users having a high level of skill in application, or a high frustration tolerance.
Some industries have developed specific products to deal with such pressure sensitive application problems. The sign industry uses large quantities of pressure sensitive vinyl, and markets an auxiliary product called "application fluid". This is essentially a slippery fluid wiped or sprayed on the tacky adhesive side of the vinyl so one can temporarily make the adhesive non-tacky for ease of placement and creation of fewer air bubbles. This fluid is a messy solution for use on non-porous substrate. Other companies have addressed the drawbacks of pressure sensitive adhesives by cutting the tack and/or peel values of the adhesives. This can be accomplished in a number of ways, such as by including additives like waxes, lubricants, and silicone. Another approach is to include fillers and particulates (e.g., microspheres) in the adhesive coating to roughen up the surface of the adhesive, thereby cutting down on initial surface contact area. It is easily appreciated that most pressure sensitive adhesive applications call for permanent, reliable bonding. The techniques for achieving less tack are unreliable and the effectiveness of the coating is unpredictable batch to batch, due to variations in coating parameters, such as oven temperature, humidity, raw materials, etc. Above all, it must be noted that in general the lower the initial tack, the lower the chances for high permanent bonding.